In general, offshore drilling rigs are used to extract and process oil and natural gas. An offshore drilling rig typically consists of a platform supported by one or more legs that engage the sea floor. When first installed, the platform is typically towed to the installation location with the platform legs fully raised up. Once the rig is in location, the supporting legs are lowered to the sea floor by an elevating unit. The supporting legs are then further extended by the elevating unit to raise the platform above the surface of the water for operation, so as to minimize the effects of the surface conditions on the platform itself.
Typically, there is an elevating unit on each corner, or ‘chord’, of each leg. Each elevating unit consists of multiple drivetrains housed in a structure attached to a hull of the platform. The elevating unit drivetrains are powered by vertically stacked gear motors during raising operations. The gear motors act as electrical brakes during lowering operations with generated energy dissipated in a resistor bank. The loads of each leg of the rig are measured from the current passing through the motors. The elevating unit and jacking legs operate as a rack and pinion style system. Once the monitored current indicates that the load is equally distributed amongst the legs, the gear motors are disabled and gear motor brakes are engaged to hold the static loads transmitted through the rack and pinion system. At this point, the load can no longer be monitored through the gear motor current.
The loads distributed through the various drivetrains and passing through to the gear motor brakes can become imbalanced as a result of variations in motor and brake performance during jacking operations, and due to external load factors such as hull loading, drilling, wave interactions and sea bed movement. In order to extend the life and wear characteristics of the drivetrain and gear motor components within the jacking system, and to minimize any safety concerns, the static loads passing through each drivetrain and gear motor are typically balanced at regular intervals (e.g., monthly).
In the past, leg load balancing has been performed manually at regular intervals. However, because of the unpredictability of the forces acting on the legs, one or more of the drivetrains within a leg could become highly overloaded during a regular interval. As a result, the rig could potentially be damaged. In addition, if one or more of the elevating units within a leg becomes significantly overloaded, there may be a large sudden impact load applied to motors within the elevating unit when the brake is released, which could damage the motor and associated geartrain.
Due to this potential imbalanced loading of the legs, chords and motors, methods and systems for monitoring the load in each leg were established. In one example, strain gauges are mounted to multiple shafts of the gearbox of the jacking drive system. These strain gauges are wired through holes that pass through the inside of the shafts, and extend to the shaft end, where a slip ring is mounted to provide power to the strain gauges and to log the measured data.
There are several shortcomings and downfalls to this type of system. Slip rings are heavy and the shaft must be altered to mount the slip ring properly. The shaft also requires alterations to allow wires to pass through the center of the shaft to connect the strain gauges to a location where the slip ring can be mounted. Further, junction boxes, signal conditioning and additional support hardware take up space, and require a plurality of wires to connect the system, all of which can be prone to breaking, or coming loose under vibrations.
Thus, there is a need for a wireless continuous monitoring system and method to monitor the load in each leg of an offshore drilling rig when the brake is engaged.